WASHINGTON, Nov. 19 — Federal prosecutors have issued grand jury subpoenas to some of the Blackwater employees present at a Sept. 16 shooting in Baghdad in which the company’s security personnel killed 17 Iraqi civilians, lawyers in the case and government officials briefed on the matter said Monday.

The opening of the grand jury inquiry is a significant step in the case because it indicates that prosecutors believe that there is enough evidence of wrongdoing to warrant a formal criminal investigation.

Officials cautioned that the decision to begin a grand jury inquiry did not mean that prosecutors had decided to charge anyone with a crime in what they said was a legally complex case. Some government lawyers have expressed misgivings about whether a federal law exists that would apply to the actions Blackwater employees are accused of committing.

The officials, who spoke on the condition of anonymity because they were discussing grand jury matters, would not say exactly how many subpoenas had been issued, but they said the subpoenas were mainly to Blackwater employees who were at the scene of the shooting but did not fire their weapons. The prosecutors are also seeking company records compiled at the time of the shooting as well as employee work histories and military service files.

The grand jury inquiry in Washington was first reported Monday by ABC News on the network’s Web site.

A spokesman for the Justice Department would not comment on whether prosecutors had convened a grand jury in the case. It was not known whether Attorney General Michael B. Mukasey approved the decision, but it would be unusual for prosecutors to take such a step in a high-profile case without advising the attorney general.

Blackwater’s spokeswoman, Anne E. Tyrrell, said that she could not confirm that a grand jury investigation was under way and that she would have no comment on any federal inquiry involving the company. She did say, however, that it would not be unexpected that federal prosecutors would convene a grand jury to support the F.B.I. investigation.

“It should come as no surprise that this might happen when the F.B.I. is investigating such a matter,” she said.

She added, “If official findings conclude that someone was complicit in wrongdoing, we will support holding that person accountable.”

The existence of a formal criminal inquiry intensifies the pressure on Blackwater, which is seeking to salvage its reputation and preserve its lucrative security contracts with the federal government. The company has earned nearly $1 billion in federal contracts since 2001 and is currently operating under a $1.2 billion contract with the State Department. That contract expires next spring, and State Department officials have said that renewal depends in part on the outcome of the investigations.

Federal agents investigating the events of Sept. 16 have found that at least 14 of the 17 killings were unjustified and violated deadly-force rules in effect for security contractors in Iraq, said civilian and military officials briefed on the case.

Only 5 of the 19 guards believed to be in the convoy that morning fired their weapons, the officials said. Prosecutors are gradually narrowing the case to the likely targets of the inquiry, the employees who fired upon the Iraqis, including an unidentified guard referred to by investigators as “turret gunner No. 3” who was responsible for a number of deaths.

The ABC News report, citing sworn statements given by Blackwater employees to State Department investigators, suggested that Blackwater guards had given contradictory accounts of the chaotic violence in the episode, in Nisour Square in central Baghdad.

One guard reportedly told the authorities that he did not observe any hostile activity directed toward the convoy before the shooting. But another is said to have reported seeing individuals in Iraqi police uniforms firing at the convoy before Blackwater guards opened fire. It is not known whether these Blackwater guards were among those who received subpoenas.

Criminal investigators found no evidence to support assertions by Blackwater employees that they were fired upon by Iraqi civilians, contradicting initial assertions by Blackwater officials who said that company employees fired in self-defense and that several vehicles were badly damaged by hostile gunfire.

The F.B.I. inquiry has faced a number of hurdles. Agents did not arrive in Baghdad until several weeks after the shooting and were thus hampered in efforts to reconstruct the scene. Federal investigators have impounded several vehicles, but some officials have said they may reveal little forensic evidence.

The State Department, which employs Blackwater to guard its diplomats and other high-level civilians in Iraq, interviewed most of the Blackwater agents involved in the shooting under grants of immunity. Those statements are off limits to the F.B.I. and cannot be used in any prosecution.

In addition, government officials said a number of Blackwater guards declined to cooperate with the F.B.I. and had retained lawyers.